The present invention relates in general to nonlinear optical signal processing and more particularly concerns novel apparatus and techniques for nonlinearly processing optical signals with crystals having a sequence of rotational twin planes to appreciably enhance output power with a compact structure that is relatively easy and inexpensive to fabricate and is capable of handling relatively high levels of power over a wide range of wavelengths. Crystals with rotational twin planes are normally considered undesirable by those skilled in the art who prefer single domain crystals. The invention uses these crystals with rotational twin planes to provide an essentially monochromatic optical source tunable over a range greater than five octaves, believed to be a continuous tuning range at optical and near optical wavelengths significantly greater than that previously achieved.
In optical systems using laser sources it is often desirable to transfer the high spectral brightness of a laser at one wavelength to another wavelength different from the first (pump) wavelength. Typical mixing media include nonlinear optical materials such as transparent crystals and atomic vapors using the property of phase matching. The optical wave generated in the nonlinear medium must be in a proper phase relationship to the primary optical beams which excite the nonlinear effect.
Typical nonlinear interactions include frequency doubling, generation of difference frequencies, up-conversion and parametric oscillation. A difficulty with the prior art techniques is the relatively low power provided by the nonlinear devices.
Bloembergen (U.S. Pat. No. 3,384,433) and other prior art typically disclose that phase matching may be achieved by making crystals with the refractive index or nonlinear coefficient or slabs of material possessing specific lattice orientation periodically disposed along the axis of propagation of the optical beams. The prior art fails to disclose practically acceptable methods of obtaining or manufacturing the periodic material.
Accordingly, it is an important object of this invention to provide practical methods and means for nonlinearly processing optical signals.
It is a more specific object of the invention to provide improved methods and means for nonlinearly processing optical signals.
It is a further object of the invention to achieve one or more of the preceding objects while providing higher power output.
It is a further object of the invention to achieve one or more of the preceding objects with inexpensive crystals of a type generally regarded as scrap capable of handling relatively high optical power levels.
It is a further object of the invention to achieve one or more of the preceding objects by using crystals having a sequence of rotational twin planes.